Semiconductor chips are often electrically connected to external circuitry using wire bonds. Wires are connected between contact pads on the chip surface and the desired external circuitry. However, the wire length adds unwanted resistance and inductance to the system, which can negatively affect the performance of the chip. This is especially true of chips designed for use at higher frequencies. Furthermore, the contact pads on the chip surface increase the area of each chip, which decreases the number of chips that can be manufactured on a single wafer. Wire bonding is also an expensive and time-consuming assembly process.
In some applications, wire bonding is replaced with conductive, metal-coated vias that run through the wafer to connect a contact on the front of the chip to a contact pad on the back of the chip. A contact pad is generally larger than a contact, since contact pads are usually used to connect to larger, external circuitry. By placing contact pads on the back of the chip, the over-all surface area of the chip can be reduced since these contact pads can overlap the device on the front of the chip. Furthermore, the contact pads on the back of a chip can be directly connected to external circuitry by ball bump or other technologies, thus eliminating the need for wire-bonding altogether.
Metal-coated vias work well when the via is large. However, it is difficult to coat the walls of a very small via—a via that is less than 50-80 microns wide at its widest. Due to the steep walls and the diminutive opening of a small via, it is difficult to get enough metal into the via and onto its walls to ensure a solid, reliable contact.